The world’s major central banks unveiled a new strategy Wednesday to keep Europe’s debt crisis from choking off global lending, a dramatic step that comes as the availability of credit for businesses and consumers has shown signs of freezing up.

In an action that recalls the depths of the U.S. financial crisis three years ago, when global central bankers took coordinated steps to stem a worldwide panic, the U.S. Federal Reserve and five of its sister institutions agreed to supply one another with unlimited amounts of each country’s money at a reduced cost. Most immediately, this initiative means the European Central Bank can pump dollars into banks in the troubled euro zone at low interest rates.

The announcement, coupled with a separate move by China’s central bank to loosen bank lending, sent stocks soaring on Wall Street and in other financial capitals. The Dow Jones industrial average rose 490 points, or 4.2 percent, the strongest gain in more than two years.

The Fed and the ECB, along with central banks in Britain, Canada, Japan and Switzerland, were responding to the difficulties European banks face in raising the money they need to conduct routine business. This challenge has become acute in the past two weeks.

Investors have grown wary of lending money to European banks, fearing that these firms could face vast losses on their holdings of bonds issued by cash-short European governments. Without access to dollars, the banks could start canceling loans — and there are signs they have already begun doing so. That would further undermine Europe’s already-weak economy, making the debt and banking crises that much worse.

“Lower lending to small businesses in France — and across Europe — is hitting at the ability to bring down unemployment rates and stimulate growth,” said Olivier Pastre, a professor of economics at the University of Paris VIII.

The world’s most powerful central banks are stepping in and using their unlimited ability to print money and lend it across national borders to try to arrest that dangerous cycle. The central banks are using what are called “swap lines” to exchange their respective currencies. While this arrangement has been used intermittently since 2007, the new policy makes dollars available more cheaply than before.

Dollars are crucial for business in many countries, particularly in Europe, where banks do extensive lending in dollars even though their home countries primarily use the euro or other currencies. But unlike U.S. banks, which have constant access to the Fed and its unlimited capacity to supply dollars, foreign banks must rely on private markets to supply them the dollars they need.

In a climate of fear, when the solvency of those banks is in question, dollars become hard to come by. That happened in 2007 and 2008, when the Fed, the ECB and other central banks first established the swap lines.

The policy works like this: The Fed lends dollars to, say, the ECB, in exchange for euros of comparable value. The ECB pays interest and lends the dollars to banks in the euro area that have obligations in dollars but are temporarily unable to borrow dollars to meet them. After a fixed period, the Fed gets its dollars back and the ECB gets its euros back.

The swap lines pose little risk to the U.S. taxpayer, Fed officials have said, because the Fed is doing business with foreign central banks viewed as trustworthy. Those foreign central banks, in turn, take the risk of loss if the banks they are lending to go under.

“The purpose of these actions is to ease strains in financial markets and thereby mitigate the effects of such strains on the supply of credit to households and businesses and so help foster economic activity,” said a joint statement Wednesday morning from the six central banks.

The latest emergency measure could prove more symbolic than substantive. The Fed already has made available unlimited dollars to other leading central banks, and $2.4 billion of such lending was outstanding as of last week. But such lending may now expand as the interest rate drops. (To access dollars from the Fed, central banks will now pay an interest rate that is five-tenths of a percentage point above a private overnight lending rate. Previously, central banks paid one percentage point above the overnight rate.)

Broadly, the new move offers reassurance to unsettled markets, a sense that the world’s central bankers have a common purpose and deep resolve even if Europe’s elected leaders seem divided and behind the curve.

Also helping to boost confidence, China’s central bank eased a key regulation to allow Chinese banks to lend more money. In reducing by half a percentage point the amount of money commercial banks must keep in reserve, the People’s Bank of China signaled that its focus is shifting from inflation to economic growth.

In Europe, banks have seen their funding costs rise to the highest levels since the collapse of Lehman Brothers in late 2008, weighing on the region’s growth.

“For banks, it is a jolt comparable to the petroleum shocks. We will never come back to the situation as it was before the crisis,” Baudouin Prot, the new president of BNP Paribas, the largest bank in the euro zone, said Wednesday in an interview with Le Monde.

In Germany, some of the country’s powerhouse banks have significantly curtailed the amount of money they allow to flow across their borders.

“Lending to foreign banks has gone down,” said Klaus Deutsch, an economist at Deutsche Bank Research.

And the impact has not been confined to Europe. Confronting a growing credit crunch, French banks, in particular, are retrenching globally. BNP Paribas, for instance, recently pulled out of a $2 billion refinancing deal for Australian media giant Seven West, while Societe Generale shied away from a $15 billion loan in the United Technologies acquisition of Goodrich, according to Reuters.

Europe’s largest financial firms represent some of the biggest lenders to infrastructure projects in the developing world, particularly in the energy sector.

Faiola reported from London. Correspondents Michael Birnbaum in Berlin and Edward Cody in Paris and staff writer Howard Schneider in Washington contributed to this report.

Anthony Faiola is The Washington Post’s South America/Caribbean bureau chief. Since joining the paper in 1994, he has served as bureau chief in Berlin, London, Tokyo, Buenos Aires and New York. He has also covered global economics from Washington.